Zoom lens and image pickup apparatus equipped with same

ABSTRACT

A zoom lens is composed, in order from the object side thereof, of a front side lens unit having a negative refracting power at the wide angle end and a rear side lens unit having a positive refracting power at the wide angle end. The front side lens unit includes a first lens unit located closest to the object side and having a positive refracting power and a second lens unit located on the image side of the first lens unit and having a negative refracting power. The distance between the first lens unit and the second lens unit is larger at the telephoto end than at the wide angle end. The rear side lens unit includes a third lens unit located closer to the object side at the telephoto end than at the wide angle end and having a positive refracting power, the distance between the third lens unit and the second lens unit being smaller at the telephoto end than at the wide angle end. The third lens unit satisfies the following condition: 
       0.01&lt; f   3   /f   t &lt;0.16  (1).

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a divisional of U.S. patent application Ser.No. 12/287,636 filed on Oct. 9, 2008, which is based upon and claims thebenefit of priority from the prior Japanese Patent Application Nos.2007-270457 filed on Oct. 17, 2007 and 2007-270553 filed on Oct. 17,2007; the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates a zoom lens and an image pickup apparatusequipped with the same.

2. Description of the Related Art

In recent years, digital cameras that pick up an image of an objectusing an image pickup element such as a CCD or CMOD sensor have replacedfilm cameras to become the mainstream. Furthermore, various categoriesof digital cameras ranging from popular-priced compact types to advancedtypes for professionals have been developed. Among these is a type ofcamera having a lens with a zoom ratio of about 10 or higher that isaccommodated in the camera body when not in use so that it becomescompact.

Zoom lenses having a zoom ratio of about 10 or higher include, forexample, a type of zoom lens having, in order from the object sidethereof, a first lens unit having a positive refracting power, a secondlens unit having a negative refracting power and a third lens unithaving a positive refracting power as know from Japanese PatentApplication Laid-Open Nos. 11-5244, 11-6958, 2006-171055, 2007-10695 and2007-3554.

SUMMARY OF THE INVENTION

A first zoom lens according to one aspect of the present inventionconsists, in order from the object side thereof, of:

a front side lens unit having a negative refracting power at the wideangle end; and

a rear side lens unit having a positive refracting power at the wideangle end, wherein

the front side lens unit comprises a first lens unit located closest tothe object side and having a positive refracting power and a second lensunit located on the image side of the first lens unit and having anegative refracting power,

the distance between the first lens unit and the second lens unit islarger at the telephoto end than at the wide angle end,

the rear side lens unit comprises a third lens unit located closer tothe object side at the telephoto end than at the wide angle end andhaving a positive refracting power, the distance between the third lensunit and the second lens unit being smaller at the telephoto end than atthe wide angle end, and the third lens unit satisfies the followingcondition:

0.01<f ₃ /f _(t)<0.16  (1)

where f₃ is the focal length of the third lens unit, and f_(t) is thefocal length of the entire zoom lens system at the telephoto end.

A second zoom lens according to another aspect of the present inventionconsists, in order from the object side thereof, of:

a front side lens unit having a negative refracting power at the wideangle end; and

a rear side lens unit having a positive refracting power at the wideangle end, wherein

the front side lens unit comprises a first lens unit located closest tothe object side and having a positive refracting power and a second lensunit located on the image side of the first lens unit and having anegative refracting power, the distance between the first lens unit andthe second lens unit being larger at a telephoto end than at the wideangle end,

the rear side lens unit comprises a third lens unit located closer tothe object side at the telephoto end than at the wide angle end andhaving a positive refracting power, the distance between the third lensunit and the second lens unit being smaller at the telephoto end than atthe wide angle end, and

the first lens unit satisfies the following condition:

0.15<f ₁ /f _(t)<0.50  (2)

where f₁ is the focal length of the first lens unit, and f_(t) is thefocal length of the entire zoom lens system at the telephoto end.

A third zoom lens according to still another aspect of the presentinvention consists, in order from an object side thereof, of:

a front side lens unit having a negative refracting power at a wideangle end; and

a rear side lens unit having a positive refracting power at the wideangle end, wherein

the front side lens unit consists of a first lens unit having a positiverefracting power and a second lens unit having a negative refractingpower,

the rear side lens unit consists of a third lens unit having a positiverefracting power and a rear lens unit on the image side of the thirdlens unit,

during zooming from the wide angle end to the telephoto end, distancesbetween the first lens unit, the second lens unit, the third lens unitand the rear lens unit respectively change,

the distance between the first lens unit and the second lens unit islarger at the telephoto end than at the wide angle end,

the distance between the second lens unit and the third lens unit issmaller at the telephoto end than at the wide angle end,

the distance between the third lens unit and the rear lens unit at thetelephoto end is different from that at the wide angle end,

the first lens unit is located closer to the object side at thetelephoto end than at the wide angle end,

the third lens unit is located closer to the object side at thetelephoto end than at the wide angle end, and

the zoom lens satisfies the following condition:

0.10<(ΣD _(1-R))/f _(t)<0.28  (10)

where ΣD_(1-R) is the sum of the thicknesses, on the optical axis, ofthe first lens unit, the second lens unit, the third lens unit and therear lens unit, the thickness of each lens unit referring to the actualdistance from the object side surface of the lens located closest to theobject side in that lens unit to the image side surface of the lenslocated closest to the image side in that lens unit, and f_(t) is thefocal length of the entire zoom lens system at the telephoto end.

A fourth zoom lens according to still another aspect of the presentinvention consists, in order from the object side thereof, of:

a front side lens unit having a negative refracting power at a wideangle end; and

a rear side lens unit having a positive refracting power at the wideangle end, wherein

the front side lens unit consists of a first lens unit having a positiverefracting power and a second lens unit having a negative refractingpower,

the rear side lens unit consists of a third lens unit having a positiverefracting power and a rear lens unit on the image side of the thirdlens unit,

during zooming from the wide angle end to the telephoto end, distancesbetween the first lens unit, the second lens unit, the third lens unitand the rear lens unit respectively change,

the distance between the first lens unit and the second lens unit islarger at the telephoto end than at the wide angle end,

the distance between the second lens unit and the third lens unit issmaller at the telephoto end than at the wide angle end,

the distance between the third lens unit and the rear lens unit at thetelephoto end is different from that at the wide angle end,

the first lens unit is located closer to the object side at thetelephoto end than at the wide angle end,

the third lens unit is located closer to the object side at thetelephoto end than at the wide angle end, and

the zoom lens satisfies the following condition:

0.05<(ΣD ₁₂ R)/f _(t)<0.19  (11)

where ΣD_(12R) is the sum of the thicknesses, on the optical axis, ofthe first lens unit, the second lens unit and the rear lens unit, thethickness of each lens unit referring to an actual distance from anobject side surface of the lens located closest to the object side inthat lens unit to an image side surface of the lens located closest tothe image side in that lens unit, and f_(t) is the focal length of theentire zoom lens system at the telephoto end.

An image pickup apparatus according to still another aspect of thepresent invention comprises:

a zoom lens; and

an image pickup element disposed on the image side of the zoom lens thatconverts an image formed by the zoom lens into an electrical signal,wherein

the zoom lens is at least any one of the above-described first to fourthzoom lenses.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B, 1C, 1D and 1E are cross sectional views taken along anoptical axis showing the configuration of a first embodiment of the zoomlens according to the present invention in the state in which the zoomlens is focused on an object point at infinity, where FIG. 1A shows thestate at the wide angle end, FIG. 1B shows a first intermediate focallength state and FIG. 1C shows a second intermediate focal length state,FIG. 1D shows a third intermediate focal length state, and FIG. 1E showsthe state at the telephoto end;

FIGS. 2A, 2B, 2C, 2D and 2E are cross sectional views similar to FIGS.1A, 1B, 1C, 1D and 1E respectively, showing the configuration of asecond embodiment of the zoom lens according to the present invention;

FIGS. 3A, 3B, 3C, 3D and 3E are cross sectional views similar to FIGS.1A, 1B, 1C, 1D and 1E respectively, showing the configuration of a thirdembodiment of the zoom lens according to the present invention;

FIGS. 4A, 4B and 4C are diagrams showing spherical aberration,astigmatism, distortion and chromatic aberration of magnification in thefirst embodiment in the state in which the zoom lens is focused on anobject point at infinity, where FIG. 4A shows aberrations at the wideangle end, FIG. 4B shows aberrations in the first intermediate focallength state and FIG. 4C shows aberrations in the second intermediatefocal length state;

FIGS. 5D and 5E are diagrams showing spherical aberration, astigmatism,distortion and chromatic aberration of magnification in the firstembodiment in the state in which the zoom lens is focused on an objectpoint at infinity, where FIG. 5D shows aberrations in the thirdintermediate focal length state and FIG. 5E shows aberrations at thetelephoto end;

FIGS. 6A, 6B and 6C are diagrams similar to FIGS. 4A, 4B and 4C showingaberrations in the second embodiment in the state in which the zoom lensis focused on an object point at infinity;

FIGS. 7D and 7E are diagrams similar to FIGS. 5D and 5E showingaberrations in the second embodiment in the state in which the zoom lensis focused on an object point at infinity;

FIGS. 8A, 8B and 8C are diagrams similar to FIGS. 4A, 4B and 4C showingaberrations in the third embodiment in the state in which the zoom lensis focused on an object point at infinity;

FIGS. 9D and 9E are diagrams similar to FIGS. 5D and 5E showingaberrations in the third embodiment in the state in which the zoom lensis focused on an object point at infinity;

FIGS. 10A, 10B and 10C are cross sectional view of the lenses accordingto the first to third embodiments respectively in the collapsed state;

FIG. 11 is a diagram illustrating correction of distortion;

FIG. 12 is a front perspective view showing an outer appearance of adigital camera equipped with a zoom lens according to the presentinvention;

FIG. 13 is a rear perspective view of the digital camera;

FIG. 14 is a cross sectional view of the digital camera; and

FIG. 15 is a block diagram of an internal circuit of a principal portionof the digital camera.

DETAILED DESCRIPTION OF THE INVENTION

The basic configuration of the first zoom lens according to the presentinvention is that the zoom lens is composed, in order from the objectside thereof, of:

a front side lens unit having a negative refracting power at the wideangle end; and

a rear side lens unit having a positive refracting power at the wideangle end, wherein

the front side lens unit includes a first lens unit having a positiverefracting power located closest to the object side, and a second lensunit having a negative refracting power disposed on the image side ofthe first lens unit,

the distance between the first lens unit and the second lens unit islarger at the telephoto end than at the wide angle end, and

the rear side lens unit includes a third lens unit having a positiverefracting power, the distance between the third lens unit and thesecond lens unit being smaller at the telephoto end than at the wideangle end.

By this configuration, the front side lens unit and the rear side lensunit forms a kind of retrofocus-type lens configuration at the wideangle end, which is advantageous in achieving an adequately large angleof field at the wide angle end.

In addition, by increasing the distance between the first lens unit andthe second lens unit, it becomes easy to provide the second lens unithaving a negative refracting power with an adequate magnificationchanging function.

By making the third lens unit having a positive refracting power in therear side lens unit closer to the second lens unit, it becomes also easyto provide the third lens unit with a magnification changing function.

Furthermore, in the first zoom lens according to the present invention,in the above-described zoom lens, the third lens unit has a refractingpower that satisfies the following conditional expression (1), and thethird lens unit is moved in such a way as to be located closer to theobject side at the telephoto end than at the wide angle end:

0.01<f ₃ /f _(t)<0.16  (1)

where f₃ is the focal length of the third lens unit, and f_(t) is thefocal length of the entire zoom lens system at the telephoto end.

By making the refracting power of the third lens unit relative to therefracting power of the entire zoom lens system at the telephoto endlarger as compared to prior arts, it becomes easy to provide the rearside lens unit with an adequate positive refracting power at the wideangle end. This is advantageous in making the entire length of the zoomlens at the wide angle end shorter and in making the focal length at thewide angle end shorter.

Furthermore, displacing the third lens unit having a positive refractingpower toward the object side is advantageous in providing the third lensunit with a sufficient magnification changing function and in making theentire length of the zoom lens at the telephoto end shorter.

Conditional expression (1) regulates the refracting power of the thirdlens unit.

By designing the third lens unit in such a way that the lower limit ofconditional expression (1) is not exceeded, it becomes easy to reducethe number of lenses required to suppress generation of aberrations(primarily, spherical aberration) in the third lens unit. This isadvantageous in reducing the size.

By designing the third lens unit in such a way that the upper limit ofconditional expression (1) is not exceeded, it becomes easy to designthe third lens unit in such a way as to provide adequate magnificationchange. This is advantageous in reducing the size and in making the zoomratio high.

Furthermore, the first lens unit may be designed in such a way as to belocated closer to the object side at the telephoto end than at the wideangle end. This is advantageous in increasing the zoom ratio and inreducing the entire length at the wide angle end and in other respects.

In the second zoom lens according to the present invention, the firstlens unit in the above-described basic zoom lens is configured to have arefracting power that satisfies the following conditional expression(2), and the first lens unit is moved in such a way as to be locatedcloser to the object side at the telephoto end than at the wide angleend:

0.15<f ₁ /f _(t)<0.50  (2)

where f₁ is the focal length of the first lens unit, and f_(t) is thefocal length of the entire zoom lens system at the telephoto end.

Thus, the refracting power of the first lens unit relative to therefracting power of the entire zoom lens system at the telephoto end ismade higher as compared to prior arts. This provides further advantagesin providing adequate magnification change by the second lens unithaving a negative refracting power. In addition, by locating the firstlens unit closer to the object side at the telephoto end than at thewide angle end, the entire length of the zoom lens at the wide angle endcan be made smaller. This is also advantageous in making the diameter ofthe first and second lens units smaller. This also facilitates reductionin variations of the F-number necessitated by an increased zoom ratio.

Conditional expression (2) regulates the refracting power of the firstlens unit.

By designing the first lens unit in such a way that the lower limit ofconditional expression (2) is not exceeded, it becomes easy to reducethe number of lenses required to reduce generation of aberrations(primarily, spherical aberration at the telephoto end) in the first lensunit. This is advantageous in making the size of the first lens unitsmaller.

By designing the first lens unit in such a way that the upper limit ofconditional expression (2) is not exceeded, it becomes easy to provideadequate magnification change by the second lens unit. This isadvantageous in achieving a high zoom ratio while making the change inthe distance between the first lens unit and the second lens unit smalland in making the entire length of the zoom lens at the telephoto endsmaller.

Furthermore, it is preferred that the third lens unit be located closerto the object side at the telephoto end than at the wide angle end. Thisis advantageous in providing the third lens unit with an adequatemagnification changing function.

It is more preferred that the feature of the first zoom lens and thefeature of the second zoom lens be both adopted in order to achievebetter balance between increase in the zoom ratio and reduction in theentire length of the zoom lens.

In either one of the above described zoom lenses, it is preferred thatone or some of the following features be adopted.

It is preferred that the second lens unit satisfy the followingcondition:

0.01<|f ₂ |/f _(t)<0.10  (3)

where f₂ is the focal length of the second lens unit.

Conditional expression (3) specifies preferred refracting powers for thesecond lens unit from the viewpoint of achieving good balance betweenthe compactness and optical performance.

By designing the second lens unit in such a way that the lower limit ofconditional expression (3) is not exceeded, it becomes easy to suppressgeneration of spherical aberration and curvature of field in the secondlens unit.

Designing the second lens unit in such a way that the upper limit ofconditional expression (3) is not exceeded is advantageous in providingthe second lens unit with an adequate magnification changing functionand in reducing the size of the lens barrel with reduction in the entirelength of the zoom lens.

It is also preferred that the first lens unit and the second lens unitsatisfy the following condition in relation to each other:

1.26<ΣD1/ΣD2<3.00  (4)

where ΣD1 is the thickness (i.e. the length along the optical axis) ofthe first lens unit on the optical axis, and ΣD2 is the thickness of thesecond lens unit on the optical axis, the thickness of each lens unit onthe optical axis referring to the actual distance from the object sidesurface of the lens located closest to the object side in the lens unitto the image side surface of the lens located closest to the image sidein the lens unit.

Conditional expression (4) specifies preferred ratios of the thicknessof the first lens unit to the thickness of the second lens unit.

To achieve an adequately large angle of field at the wide angle end anda high zoom ratio, it is preferred that spherical aberration at zoompositions near the telephoto end and curvature of field at zoompositions near the wide angle end be corrected excellently.

Designing the zoom lens in such a way that the lower limit ofconditional expression (4) is not exceeded so as to make the thicknessof the second lens unit appropriately small and make the thickness ofthe first lens unit on the optical axis appropriately large isadvantageous in providing the first lens unit with an adequaterefracting power, achieving an adequate effective diameter at the wideangle end while achieving a wide angle of field, correcting sphericalaberration at zoom positions near the telephoto end and correctingcurvature of field at zoom positions near the wide angle end.

Designing the zoom lens in such a way that the upper limit ofconditional expression (4) is not exceeded so as to prevent the firstlens unit from becoming large is advantageous in reducing the size.Alternatively, the second lens unit may be designed to have anappropriately large thickness, which makes it easy to provide the secondlens unit with an adequate refracting power and achieve adequate opticalperformance.

It is preferred that the second lens unit be composed of two negativelenses and one positive lens.

Thus, the negative refracting power of the second lens unit isdistributed to the two negative lenses, and aberrations are cancelled bythe one positive lens. This is advantageous in slimming the second lensunit and reducing variations in aberrations even though the second lenshas a magnification changing function.

It is also preferred that a fourth lens unit having a positiverefracting power be provided on the image side of the third lens unitand an iris stop be provided between the second lens unit and the fourthlens unit.

By providing the fourth lens unit, it becomes easy to locate the exitpupil at a farther position from the image plane, which is advantageousin achieving adequate peripheral light quantity in the case where animage pickup element such as a CCD or a CMOS sensor is used with thezoom lens.

It is also preferred that the zoom lens be a four-unit zoom lens.

By configuring the zoom lens as a four-unit zoom lens with apositive-negative-positive-positive refracting power arrangement (inorder from the object side,) of the lens units, load on a mechanism fordriving lens units can be made small.

It is also preferred that during zooming from the wide angle end to thetelephoto end,

the first lens unit move in such a way as to be located closer to theobject side at the telephoto end than at the wide angle end,

the second lens unit move,

the third lens unit move in such a way as to be located closer to theobject side at the telephoto end than at the wide angle end,

the fourth lens unit move, and

the iris stop move in such a way as to be located closer to the objectside at the telephoto end than at the wide angle end.

By moving these lens units, it becomes easy to reduce variations inaberrations with an increase in the zoom ratio and to arrangedistribution of amounts of movement of the lens units.

By moving the iris stop in such a way that it is located closer to theobject side at the telephoto end than at the wide angle end, the size ofthe third lens unit can easily be made small, and it becomes preferablyeasy to provide an adequate degree of freedom in designing the movementranges of the second lens unit and the third lens unit.

Furthermore, the movements of the respective lens unit during zoomingfrom the wide angle end to the telephoto end may be designed as follows.The first lens unit may be moved only toward the object side or along alocus that is convex toward the object side or the image side. Thesecond lens may be moved only toward the object side or along a locusthat is convex toward the object side or the image side. The third lensmay be moved only toward the object side or along a locus that is convextoward the object side. The fourth lens unit may be moved in such a wayas to be located closer to the object side or the image side at thetelephoto end than at the wide angle end.

It may be moved monotonously or along a locus that is convex toward theobject side or the image side.

It is also preferred that the fourth lens unit having a positiverefracting power be moved in such a way as to be located closer to theimage side at the telephoto end than at the wide angle end.

By providing the fourth lens unit with a magnification increasingfunction, the movement amounts of the lens units that move duringzooming can be made smaller. This is advantageous in achieving both asufficiently high zoom ratio and reduction in the size of the lensframe.

It is preferred that the fourth lens unit be the lens unit closest tothe image side among the lens units in the zoom lens and satisfy thefollowing condition:

0.05<f ₄ /f _(t)<0.30  (5)

where f₄ is the focal length of the fourth lens unit.

Providing the fourth lens unit closest to the image side is advantageousin controlling astigmatism.

Conditional expression (5) specifies preferred refracting powers for thefourth lens unit.

It is preferred to design the fourth lens unit in such a way that thelower limit of conditional expression (5) is not exceeded therebypreventing overcorrection of astigmatism and that the upper limit ofconditional expression (5) is not exceeded to prevent undercorrectionthereof.

When some of conditional expressions (1), (2), (3), (4) and (5) aresatisfied simultaneously, it is easy to achieve favorable distributionof refracting powers to the respective lens units, which providesfurther advantages in reducing the size of the zoom lens while achievinga high zoom ratio.

It is also preferred that the zoom lens be provided with an iris stopand a shutter located between the second lens unit and the third lensunit, and the iris stop and the shutter be moved integrally with thethird lens unit and located closer to the object side at the telephotoend than at the wide angle end.

By this feature, the entrance pupil can be located at a position nearthe object side and the exit pupil can be located far from the imageplane. In addition, since the shutter is located at a position at whichthe height of off-axis rays is low, a large-size shutter unit is notrequired and the dead space associated with the movement of the irisstop and the shutter unit can be made small.

By moving the iris stop, effective correction of spherical aberrationcan be achieved, which not only is effective in improving theperformance but also enables appropriate control of the position of theentrance pupil and the position of the exit pupil. This means that theheight of off-axis rays at the wide angle end and the height of off-axisrays at the telephoto end can be well balanced, and the outer diameterof the first lens unit and the outer diameter of the lens unit closestto the image side can be made small with good balance. In particular, areduction of the outer diameter of the first lens unit at the wide angleend effectively leads also to a reduction in the size of the lens withrespect to the thickness direction (i.e. the direction along the opticalaxis). Furthermore, variations in the position of the exit pupil duringzooming can be controlled or made small. Thus, in cases where an imagepickup element such as a CCD or a CMOS sensor is used the zoom lens, theincidence angle of rays on the image pickup element can be kept withinan appropriate range, and therefore shading in the regions near thecorners of the image area can be prevented from occurring.

It is also preferred that the first lens unit have a negative lens and apositive lens.

This facilitates reduction of chromatic aberration at zoom positionsnear the telephoto end that tends to be conspicuous when the zoom lenshas a high zoom ratio.

Furthermore, if the first lens unit is configured to include a cementedlens having a negative lens and a positive lens, deterioration of theoptical performance due to relative decentering of lenses caused byassembly errors can be prevented, which is conducive to improvement ofthe throughput and cost reduction.

The negative lens and the positive lens in the first lens unit may beindependent lens components that are not cemented. In this case, it ispossible to correct spherical aberration at the telephoto end moreeffectively.

Furthermore, by constituting the first lens unit by the two lenses (i.e.the negative lens and the positive lens), the lens unit can be made morecompact with respect to the optical axis direction and the diametricaldirection.

It is also preferred that the zoom lens satisfy the followingconditions:

9<f _(t) /f _(w)<50  (6)

1.1<L _(t) /L _(w)<2.0  (7)

where f_(w) is the focal length of the entire zoom lens system at thewide angle end, L_(t) is the actual distance, on the optical axis, fromthe lens surface closest to the object side in the first lens unit tothe image plane at the telephoto end, and L_(w) is the actual distance,on the optical axis, from the lens surface closest to the object side inthe first lens unit to the image plane at the wide angle end.

Conditional expression (6) relates to the ratio of the focal length ofthe entire zoom lens system at the wide angle end and that at thetelephoto end.

It is preferred, in taking advantage of the above described features,that the zoom lens be designed to have such a high zoom ratio that thelower limit of conditional expression (6) is not exceeded.

By designing the zoom lens in such a way that the upper limit ofconditional expression (6) is not exceeded, an increase in the number oflens units or an increase in the number of lenses required for providingappropriate optical performance can be easily prevented.

Conditional expression (7) relates to the ratio of the entire length ofthe zoom lens at the wide angle end to the entire length of the zoomlens at the telephoto end.

It is preferred that the zoom lens be designed in such a way that thelower limit of conditional expression (7) is not exceeded therebyreducing the entire length of the zoom lens at the wide angle end toachieve reduction in the size with respect to the diametrical directionand facilitating achievement of a high zoom ratio by designing the zoomlens in such a way as to have an adequate overall length at thetelephoto end.

In the case where the image pickup apparatus is equipped with a flashunit, it is necessary to take care of vignetting or partial blocking offlash by the barrel of the zoom lens at the time of photographing atwide angle positions. By reducing the entire length of the zoom lens atthe wide angle end, it becomes easy to reduce unevenness in brightnessdue to vignetting of flash without disposing the flash unit at aposition far from the zoom lens. This is advantageous in reducing theoverall size of the image pickup apparatus and in achieving anadequately wide angle of field at the wide angle end.

Designing the zoom lens in such a way that the upper limit ofconditional expression (7) is not exceeded to make variations in theentire length of the zoom lens small is advantageous in reducing thesize of a lens frame for driving the zoom lens.

It is also preferred that the zoom lens satisfy the following conditionat the telephoto end:

5.6<F_(t)<16.0  (8)

where F_(t) is the F-number of the zoom lens at the telephoto end or theminimum F-number in the case where the F-number is variable.

Conditional expression (8) regulates the F-number at the telephoto end.

Designing the zoom lens in such a way that the lower limit ofconditional expression (8) is not exceeded is advantageous in reducinginfluences of spherical aberration and chromatic aberration ofmagnification that tend to be conspicuous when the zoom lens has a highzoom ratio. This is also advantageous in reducing the size of each lensunit.

By designing the zoom lens in such a way that the upper limit ofconditional expression (8) is not exceeded, influences of hand vibrationthat tend to be conspicuous when the zoom lens has a high zoom ratio canbe made smaller, which allows a mechanism for vibration reduction and/orimage processing for blur correction to be made simple and easy.

It is preferred that every lens unit included in the zoom lens have anaspheric lens surface.

If this is the case, influences of aberrations in the zoom lens having ahigh zoom ratio can be reduced in each lens unit, which is advantageousin, for example, reducing the total number of lenses included in thezoom lens.

It is preferred that the total number of lenses included in the zoomlens be in the range of 8 to 12, each of the first and third lens unitshave a negative lens and a positive lens, and the second lens unit havea positive lens and a plurality of negative lenses.

Having 8, 9, 10, 11 or 12 lenses in total is advantageous in reducingvariations in aberrations associated with a high zoom ratio, in reducingthe size of the zoom lens at the time when the lens barrel is collapsedand in reducing the cost.

Having both a positive lens and a negative lens in each of the first,second and third lens units is advantageous in reducing generation ofaberrations in each lens unit. Having a plurality of negative lenses inthe second lens unit enables the second lens unit to have an adequaterefracting power while suppressing influences of aberrations, which isadvantageous in reducing the size and in achieving a high zoom ratio.

Next, the third zoom lens according to the present invention will bedescribed.

The basic configuration of the third zoom lens according to the presentinvention is that the zoom lens is composed, in order from the objectside thereof, of:

a front side lens unit having a negative refracting power at the wideangle end, and

a rear side lens unit having a positive refracting power at the wideangle end, wherein

the front side lens unit is composed, in order from the object side, ofa first lens unit having a positive refracting power and a second lensunit having a negative refracting power,

the rear side lens unit is composed, in order from the object side, of athird lens unit having a positive refracting power and a rear lens unitlocated on the image side thereof, and

during zooming from the wide angle end to the telephoto end, thedistances between the first lens unit, the second lens unit, the thirdlens unit and the rear lens unit respectively change, where the distancebetween the first lens unit and the second lens unit is larger at thetelephoto end than at the wide angle end, the distance between thesecond lens unit and the third lens unit is smaller at the telephoto endthan at the wide angle end, the distance between the third lens unit andthe rear lens unit at the telephoto end is different from that at thewide angle end, the first lens unit is located closer to the object sideat the telephoto end than at the wide angle end, and the third lens unitis located closer to the object side at the telephoto end than at thewide angle end.

By this configuration, the front side lens unit and the rear side lensunit forms a kind of retrofocus-type lens configuration at the wideangle end, which is advantageous in achieving an adequately large angleof field at the wide angle end.

By increasing the distance between the first lens unit and the secondlens unit during zooming from the wide angle end to the telephoto end,it becomes easy to provide the second lens unit having a negativerefracting power with an adequate magnification changing function.

By bringing the third lens unit having a positive refracting power inthe rear side lens unit closer to the second lens unit during zoomingfrom the wide angle end to the telephoto end, it becomes easy to providethe third lens unit also with a magnification changing function.

By moving the first lens unit in the above described manner, it becomeseasy to reduce the entire length at the wide angle end and achieve asufficiently high zoom ratio. Thus, such a way of movement isadvantageous in achieving size reduction and high zoom ratio.

By moving the third lens unit in the above described manner, it becomeseasy to provide the third lens unit also with a magnification changingfunction, which is advantageous in achieving a sufficiently high zoomratio.

Providing the rear lens unit closest to the image side is advantageousin controlling astigmatism.

Furthermore, the third zoom lens according to the present inventionsatisfies the following condition:

0.10<(ΣD _(1-R))/f _(t)<0.28  (10)

where D_(1-R) is the sum of the thicknesses, on the optical axis, of thefirst lens unit, the second lens unit, the third lens unit and the rearlens unit, the thickness of each lens unit referring to the actualdistance from the object side surface of the lens located closest to theobject side in the lens unit to the image side surface of the lenslocated closest to the image side in the lens unit, f_(t) is the focallength of the entire zoom lens system at the telephoto end.

Conditional expression (10) provides a condition for reducing the sizeof the zoom lens at the time when the lens barrel is collapsed andachieving adequate optical performance when the zoom lens has a highzoom ratio.

It is preferred that the zoom lens is designed in such a way that thelower limit of conditional expression (10) is not exceeded so as toprovide sufficient thickness of the lens units required for aberrationcorrection.

It is preferred that the zoom lens is designed in such a way that theupper limit of conditional expression (10) is not exceeded so as torestrict the thickness, on the optical axis, of each lens unit therebyfacilitating reduction in the size in the state in which the lens barrelis collapsed.

A fourth zoom lens according to the present invention is characterizedin that in the above-described basic zoom lens, the following conditionis satisfied:

0.05<(ΣD ₁₂ R)/f _(t)<0.19  (11)

where ΣD_(12R) is the sum of the thicknesses, on the optical axis, ofthe first lens unit, the second lens unit and the rear lens unit, thethickness of each lens unit referring to the actual distance from theobject side surface of the lens located closest to the object side inthe lens unit to the image side surface of the lens located closest tothe image side in the lens unit, and f_(t) is the focal length of theentire zoom lens system at the telephoto end.

According to a known method, in order to decrease the length of the zoomlens along the optical axis at the time when the lens barrel iscollapsed, a certain lens unit(s) is retracted to a position away fromthe optical axis and kept at that position when the lens barrel of thezoom lens is collapsed.

In the zoom lens having the above-described basic configuration, thethird lens unit, which can be relatively easily made small in size, maybe retracted away from the optical axis when the lens barrel iscollapsed.

When such a collapse method is adopted, since the third lens unit isretracted, what is relevant significantly to the thickness (or thedimension along the optical axis) of the image pickup apparatus at thetime when the lens barrel is collapsed is the sum of the thickness,along the optical axis, of the first lens unit, the thickness, along theoptical axis, of the second lens unit and the thickness, along theoptical axis, of the rear lens unit.

On the other hand, if the sum of the thicknesses of the first lens unit,the second lens unit and the rear lens unit is too small relative to thefocal length f_(t) at the telephoto end, the optical performance will beaffected to a considerable degree.

By designing the zoom lens in such a way that the lower limit ofconditional expression (11) is not exceeded, it becomes easy to suppressgeneration of off-axis aberrations at the wide angle end and sphericalaberration at the telephoto end.

By designing the zoom lens in such a way that the upper limit ofconditional expression (11) is not exceeded, it becomes easy to restrictthe thicknesses, on the optical axis, of lens units among the first lensunit, the second lens unit and the rear lens unit thereby facilitatingreduction in the size in the state in which the lens barrel iscollapsed.

It is more preferred that the feature of the third zoom lens and thefeature of the third zoom lens be both adopted in order to achieve ahigh zoom ratio and a reduction in the size of the zoom lens in thestate in which the lens barrel is collapsed.

In any one of the above described zoom lenses, it is preferred that oneor some of the following features be adopted.

It is preferred that the rear lens unit have a positive refractingpower. This makes it easy to locate the exit pupil at a farther positionfrom the image plane, which facilitates reduction of color shading of animage etc. when an image pickup element is used with the zoom lens.

It is also preferred that the focal length of the rear lens unit beconstant during zooming from the wide angle end to the telephoto end. Bythis feature, the number of moving lens units is reduced, and the numberof lens frames for moving the lens units can be reduced. Thisfacilitates simplification of the structure of the zoom lens.

It is preferred that the third lens unit satisfy the followingcondition:

0.03<Σd ₃ /f _(t)<0.075  (12)

where Σd₃ is thickness of the third lens unit on the optical axis.

Conditional expression (12) specifies preferred thicknesses for thethird lens unit from the viewpoint of achieving good balance between thecompactness and optical performance.

Designing the third lens unit in such a way that the lower limit ofconditional expression (12) is not exceeded is advantageous in providingan adequate refracting power and in reducing aberrations.

Designing the third lens unit in such a way that the upper limit ofconditional expression (12) is not exceeded is advantageous in reducingthe size of the zoom lens at the time when the lens barrel is collapsed.

It is preferred that the first lens unit, the second lens unit and thethird lens unit satisfy the following condition:

0.10<Δ_(1G2G3G)/(f _(t) −f _(w))<0.77  (13)

where Δ_(1G2G3G) is the sum of the absolute values of the displacementsof the positions of the first lens unit, the second lens unit and thethird lens unit at the wide angle end from the positions of therespective lens units at the wide angle end.

The movement amounts of the lens units as well as the thicknesses of theabove-mentioned lens units tend to affect reduction in the size of thezoom lens at the time when the lens barrel is collapsed. Therefore, itis preferred, from the viewpoint of achieving good balance between sizereduction at the time when the lens barrel is collapsed and opticalperformance, that conditional expression (13) concerning the movementamounts of the first lens unit, the second lens unit and the third lensunit during zooming relative to the change in the focal length duringzooming be satisfied.

By designing the zoom lens in such a way that the lower limit ofconditional expression (13) is not exceeded, the lens units haveadequate movement amounts, and it becomes easy to make the refractingpowers of the lens units small. This is advantageous in reducingspherical aberration and curvature of field.

By designing the zoom lens in such a way that the upper limit ofconditional expression (13) is not exceeded, the movement amounts of thelens units during zooming relative to the change in the focal length canbe made moderately small, thickness of the lens frame in the collapsedstate can be made small, and the number of collapse steps can bereduced. This is advantageous in reducing the size.

It is preferred that the second lens unit is composed of two negativelenses and one positive lens.

Thus, to achieve an adequate zoom ratio, the negative refracting powerof the second lens unit can be distributed to the two negative lenses,and aberrations can be cancelled by the one positive lens. This isadvantageous in slimming the second lens unit and reducing variations inaberrations even though the second lens has a magnification changingfunction.

In this case, it is preferred that the second lens unit is composed, inorder from the object side, of a first negative lens, a positive lensand a second negative lens.

Thus, the second lens unit has a symmetrical negative-positive-negativelens arrangement, which facilitates reduction of on-axis aberrations attelephoto positions that are likely to occur with a zoom lens having ahigh zoom ratio.

Furthermore, it is preferred that the image side surface of the firstnegative lens in the second lens unit be a concave surface, the objectside surface of the second negative lens in the second lens unit be aconcave surface and the image side surface of the positive lens in thesecond lens unit be a convex surface. This is advantageous in providingan adequate space for disposing the positive lens, which in turn isadvantageous in reducing the thickness (or the length along the opticalaxis) of the second lens unit on and off the optical axis, in providingthe second unit with an adequate refracting power and in achieving goodoptical performance.

It is preferred that the zoom lens be a four-unit zoom lens. Byconfiguring the zoom lens as a four-unit zoom lens, load on a mechanismfor driving the lens units can be made small.

It is also preferred that the zoom lens be provided with an iris stopdisposed between the image side surface of the second lens unit and theimage side surface of the third lens unit, and during zooming from thewide angle end to the telephoto end,

the first lens unit move in such a way as to be located closer to theobject side at the telephoto end than at the wide angle end,

the second lens unit move,

the third lens unit move in such a way as to be located closer to theobject side at the telephoto end than at the wide angle end,

the fourth lens unit move, and

the iris stop move.

By moving these lens units, it becomes easy to reduce variations inaberrations with an increase in the zoom ratio and to arrangedistribution of movement amounts of the lens units.

By moving the iris stop in such a way that it is located closer to theobject side at the telephoto end than at the wide angle end, the size ofthe third lens unit can easily be made small, and it becomes preferablyeasy to provide an adequate degree of freedom in designing the movementranges of the second lens unit and the third lens unit.

Furthermore, the movements of the respective lens unit during zoomingfrom the wide angle end to the telephoto end may be designed as follows.

The first lens unit may be moved only toward the object side or along alocus that is convex toward the object side or the image side.

The second lens may be moved only toward the object side or along alocus that is convex toward the object side or the image side.

The third lens may be moved only toward the object side or along a locusthat is convex toward the object side.

The rear lens unit may be moved in such a way as to be located closer tothe object side or the image side at the telephoto end than at the wideangle end. It may be moved monotonously or along a locus that is convextoward the object side or the image side.

It is also preferred that the first lens unit have a negative lens and apositive lens. This facilitates reduction of chromatic aberration atzoom positions near the telephoto end that tends to be conspicuous whenthe zoom lens has a high zoom ratio.

Furthermore, if the first lens unit is configured to include a cementedlens having a negative lens and a positive lens, deterioration of theoptical performance due to relative decentering of lenses caused byassembly errors can be prevented, which is conducive to improvement ofthe throughput and cost reduction.

The negative lens and the positive lens in the first lens unit may beindependent lens components that are not cemented. In this case, it ispossible to correct spherical aberration at the telephoto end moreeffectively.

Furthermore, by constituting the first lens unit by the two lenses (i.e.the negative lens and the positive lens), the lens unit can be made morecompact with respect to the optical axis direction and the diametricaldirection.

It is also preferred that the zoom lens satisfy the followingconditions:

9<f _(t) /f _(w)<50  (6)

where f_(w) is the focal length of the entire zoom lens system at thewide angle end.

Conditional expression (6) relates to the ratio of the focal length ofthe entire zoom lens system at the wide angle end and that at thetelephoto end.

It is preferred, in taking advantage of the above described features,that the zoom lens be designed to have such a high zoom ratio that thelower limit of conditional expression (6) is not exceeded.

By designing the zoom lens in such a way that the upper limit ofconditional expression (6) is not exceeded, an increase in the number oflens units or an increase in the number of lenses required for providingappropriate optical performance can be easily prevented.

It is also preferred that the zoom lens satisfy the followingconditions:

1.1<L _(t) /L _(w)<2.0  (7)

where L_(t) is the actual distance, on the optical axis, from the lenssurface closest to the object side in the first lens unit to the imageplane at the telephoto end, and L_(w) is the actual distance, on theoptical axis, from the lens surface closest to the object side in thefirst lens unit to the image plane at the wide angle end.

Conditional expression (7) relates to the ratio of the entire length ofthe zoom lens at the wide angle end and the entire length of the zoomlens at the telephoto end.

It is preferred that the zoom lens be designed in such a way that thelower limit of conditional expression (7) is not exceeded therebyreducing the entire length of the zoom lens at the wide angle end toachieve reduction in the size with respect to the diametrical directionand facilitating achievement of a high zoom ratio by designing the zoomlens in such a way as to have an adequate overall length at thetelephoto end.

In the case where the image pickup apparatus is equipped with a flashunit, it is necessary to take care of vignetting or partial blocking offlash by the barrel of the zoom lens at the time of photographing atwide angle positions. By reducing the entire length of the zoom lens atthe wide angle end, it becomes easy to reduce unevenness in brightnessdue to vignetting of flash without disposing the flash unit at aposition far from the zoom lens. This is advantageous in reducing theoverall size of the image pickup apparatus and in achieving anadequately wide angle of field at the wide angle end.

Designing the zoom lens in such a way that the upper limit ofconditional expression (7) is not exceeded to thereby make variations inthe entire length of the zoom lens small is advantageous in reducing thesize of a lens frame for driving the zoom lens.

It is preferred that the first lens unit satisfy the followingcondition:

0.15<f ₁ /f _(t)<0.50  (2)

where f₁ is the focal length of the first lens unit.

By increasing the refracting power of the first lens unit relative tothe refracting power of the entire zoom lens system at the telephotoend, a further advantage in providing adequate magnification change bythe second lens unit having a negative refracting power is achieved.

Conditional expression (2) regulates the refracting power of the firstlens unit.

By designing the first lens unit in such a way that the lower limit ofconditional expression (2) is not exceeded, it becomes easy to reducethe number of lenses required to reduce generation of aberrations(primarily, spherical aberration at the telephoto end) in the first lensunit. This is advantageous in making the size of the first lens unitsmaller.

By designing the first lens unit in such a way that the upper limit ofconditional expression (2) is not exceeded, it becomes easy to provideadequate magnification change by the second lens unit. This isadvantageous in achieving a high zoom ratio while making the change inthe distance between the first lens unit and the second lens unit smalland in making the entire length of the zoom lens at the telephoto endsmaller.

It is preferred that the second lens unit satisfy the followingcondition:

0.01<|f ₂ |/f _(t)<0.10  (3)

where f₂ is the focal length of the second lens unit.

Conditional expression (3) specifies preferred refracting powers for thesecond lens unit from the viewpoint of achieving good balance betweenthe compactness and optical performance.

By designing the second lens unit in such a way that the lower limit ofconditional expression (3) is not exceeded, it becomes easy to suppressgeneration of spherical aberration and curvature of field in the secondlens unit.

Designing the second lens unit in such a way that the upper limit ofconditional expression (3) is not exceeded is advantageous in providingthe second lens unit with an adequate magnification changing functionand in reducing the size of the lens barrel with reduction in the entirelength of the zoom lens.

It is preferred that the third lens unit satisfy the followingcondition:

0.01<f ₃ /f _(t)<0.10  (1)

where f₃ is the focal length of the third lens unit.

By making the refracting power of the third lens unit relative to therefracting power of the entire zoom lens system at the telephoto endlarger, it becomes easy to provide the rear side lens unit with anadequate positive refracting power at the wide angle end. This isadvantageous in making the entire length of the zoom lens at the wideangle end shorter and in making the focal length at the wide angle endshorter.

Furthermore, displacing the third lens unit having a positive refractingpower toward the object side is advantageous in providing the third lensunit with an adequate magnification changing function and in making theentire length of the zoom lens at the telephoto end shorter.

Conditional expression (1) regulates the refracting power of the thirdlens unit.

By designing the third lens unit in such a way that the lower limit ofconditional expression (1) is not exceeded, it becomes easy to reducethe number of the lenses required to suppress generation of aberrations(primarily, spherical aberration) in the third lens unit. This isadvantageous in reducing the size.

By designing the third lens unit in such a way that the upper limit ofconditional expression (1) is not exceeded, it becomes easy to designthe third lens unit in such a way as to provide adequate magnificationchange, which is advantageous in reducing the size and in increasing thezoom ratio.

It is preferred that the rear lens unit satisfy the following condition:

0.05<f _(RL) /f _(t)<0.30  (14)

where f_(RL) is the focal length of the rear lens unit at the wide angleend.

Conditional expression (14) specifies preferred positive refractingpowers for the rear lens unit.

It is preferred that the rear lens unit be designed in such a way thatthe lower limit of conditional expression (14) is not exceeded tothereby prevent overcorrection of astigmatism. It is also preferred thatthe rear lens unit be designed in such a way that the upper limit ofconditional expression (14) is not exceeded to thereby prevent undercorrection.

The rear lens unit located closest to the image side may be made of aplastic material. The principal role of the rear lens unit is to controlthe position of the exit pupil appropriately thereby causing rays to beincident on an image pickup element such as a CCD or CMOS sensorefficiently. To this end, the rear lens unit is not required to have arelatively high refracting power, so long as the refracting power fallswithin the range defined by conditional expression (14). Therefore, therear lens unit may be a plastic lens made of a material having arelatively low refractive index. By using a plastic lens as the rearlens unit, the cost can be reduced, and it becomes possible to providemore inexpensive zoom lenses.

It is also preferred that the rear lens unit having a positiverefracting power be moved in such a way as to be located closer to theimage side at the telephoto end than at the wide angle end. By providingthe rear lens unit with a magnification increasing function, themovement amounts of the lens units that move during zooming can be madesmaller. This is advantageous in achieving both a sufficiently high zoomratio and reduction in the size of the lens frame.

It is also preferred that the zoom lens satisfy the following conditionat the telephoto end:

5.6<F_(t)<16.0  (8)

where F_(t) is the F-number of the zoom lens at the telephoto end or theminimum F-number in the case where the F-number is variable.

Conditional expression (8) regulates the F-number at the telephoto end.

Designing the zoom lens in such a way that the lower limit ofconditional expression (8) is not exceeded is advantageous in reducinginfluences of spherical aberration and chromatic aberration ofmagnification that tend to be conspicuous when the zoom lens has a highzoom ratio. This is also advantageous in reducing the size of each lensunit.

By designing the zoom lens in such a way that the upper limit ofconditional expression (8) is not exceeded, influences of hand vibrationthat tend to be conspicuous when the zoom lens has a high zoom ratio canbe made smaller, which allows a mechanism for vibration reduction and/orimage processing for blur correction to be made simple and easy.

It is also preferred that the first lens unit and the second lens unitsatisfy the following condition in relation to each other:

1.26<ΣD1/ΣD2<3.00  (15)

where ΣD1 is the thickness of the first lens unit on the optical axis,and ΣD2 is the thickness of the second lens unit on the optical axis,the thickness of each lens unit on the optical axis referring to theactual distance from the object side surface of the lens located closestto the object side in the lens unit to the image side surface of thelens located closest to the image side in the lens unit.

Conditional expression (15) specifies preferred ratios of the thicknessof the first lens unit to the thickness of the second lens unit.

To achieve a large angle of field at the wide angle end and a high zoomratio, it is preferred that spherical aberration at zoom positions nearthe telephoto end and curvature of field at zoom positions near the wideangle end be corrected excellently.

Designing the zoom lens in such a way that the lower limit ofconditional expression (15) is not exceeded so as to make the thicknessof the second lens unit appropriately small and make the thickness ofthe first lens unit on the optical axis appropriately large isadvantageous in providing the first lens unit with an adequaterefracting power, achieving an adequate effective diameter at the wideangle end while achieving a wide angle of field, correcting sphericalaberration at zoom positions near the telephoto end and correctingcurvature of field at zoom positions near the wide angle end.

Designing the zoom lens in such a way that the upper limit ofconditional expression (15) is not exceeded so as to prevent the firstlens unit from becoming large is advantageous in reducing the size.Alternatively, the second lens unit may be designed to have anappropriately large thickness, which makes it easy to provide the secondlens unit with an adequate refracting power and achieve adequate opticalperformance.

It is preferred that every lens unit included in the zoom lens have anaspheric lens surface. If this is the case, influence of aberrations inthe zoom lens having a high zoom ratio can be reduced in each lens unit,which is advantageous in, for example, reducing the total number oflenses included in the zoom lens.

It is preferred that the total number of lenses included in the zoomlens be in the range of 8 to 12, each of the first and third lens unitshave a negative lens and a positive lens, and the second lens unit havea positive lens and a plurality of negative lenses.

Having 8, 9, 10, 11 or 12 lenses in total is advantageous in reducingvariations in aberrations associated with a high zoom ratio, in reducingthe size of the zoom lens at the time when the lens barrel is collapsedand in reducing the cost.

Having both a positive lens and a negative lens in each of the first,second and third lens units is advantageous in reducing generation ofaberrations in each lens unit. Having a plurality of negative lenses inthe second lens unit enables the second lens unit to have an adequaterefracting power while suppressing influences of aberrations, which isadvantageous in reducing the size and in achieving a high zoom ratio.

An image pickup apparatus according to the present invention comprisesat least anyone of the above described zoom lenses, and an image pickupelement disposed on the image side of the zoom lens that converts animage formed by the zoom lens into an electrical signal.

Having a zoom lens that can advantageously achieve a high zoom ratio andbe collapsed when not in use is advantageous in reducing the size of theimage pickup apparatus.

It is preferred that the image pickup apparatus is further equipped witha flash unit.

It is also preferred that the image pickup apparatus be provided with animage processing section that performs signal processing for correctingaberrations contained in the electrical signal.

This is advantageous in further reducing the size and increasing thezoom ratio, since aberrations (e.g. distortion and chromatic aberrationof magnification) of the zoom lens are allowed to be generated.

Furthermore, it is preferred that the following condition be satisfied:

0.05<IH _(w) /f _(w)<1.00  (9)

where IH_(w) is the maximum image height at the wide angle end, andf_(w) is the focal length of the entire zoom lens system at the wideangle end.

Conditional expression (9) specifies preferred relationship between thefocal length and the maximum image height at the wide angle end.

It is preferred that the lower limit of conditional expression (9) benot exceeded to advantageously achieve an adequately large angle offield at the wide angle end.

It is preferred that the upper limit of conditional expression (9) benot exceeded to facilitate reduction of the size of the first and secondlens units with respect to the diametrical direction.

In the above described embodiments, the lens unit located closest to theimage side may be made of a plastic material. The principal role of thelens unit located closest to the image side is to control the positionof the exit pupil appropriately thereby causing rays to be incident onan image pickup element such as a CCD or CMOS sensor efficiently. Tothis end, the lens unit located closest to the image side is notrequired to have a relatively high refracting power. Therefore, the lensunit located closest to the image side may be a plastic lens made of amaterial having a relatively low refractive index. By this feature thecost can be reduced, and it becomes possible to provide more inexpensivezoom lenses and image pickup apparatuses.

It is preferred that an iris stop and a shutter be located between thesecond lens unit and the third lens unit, and the iris stop and theshutter be moved integrally with the third lens unit during zooming. Bythis feature, the entrance pupil can be located at a position near theobject side and the exit pupil can be located far from the image plane.In addition, since the shutter unit is located at a position at whichthe height of off-axis rays is low, a large-size shutter unit is notrequired and the dead space associated with the movement of the irisstop and the shutter unit can be made small.

By moving all the lens units, it is possible to effectively provide allthe lens units with magnification changing functions, and highperformance can be achieved even with a zoom lens having a large angleof field and a high zoom ratio.

By moving the iris stop, effective correction of spherical aberrationcan be achieved, which not only is effective in improving theperformance but also enables appropriate control of the position of theentrance pupil and the position of the exit pupil. This means that theheight of off-axis rays at the wide angle end and the height of off-axisrays at the telephoto end can be well balanced, and the outer diameterof the first lens unit and the outer diameter of the lens unit closes tothe image side can be made small with good balance. In particular, areduction of the outer diameter of the first lens unit at the wide angleend effectively leads also to a reduction in the size of the lens withrespect to the thickness direction (i.e. the direction along the opticalaxis). Furthermore, variations in the position of the exit pupil duringzooming can be controlled or made small. Thus, the incidence angle ofrays on a CCD or a CMOS sensor etc. can be kept within an appropriaterange, and therefore shading in the regions near the corners of theimage area can be easily prevented from occurring.

Furthermore, in order to cut or block unwanted light such as ghost andflare, a flare stop may be provided in addition to the iris stop. Theflare stop may be disposed on the object side of the first lens unit,between the first lens unit and the second lens unit, between the secondlens unit and the third lens unit, between the third lens unit and thefourth lens unit (or the rear lens unit) or between the fourth lens unit(or the rear lens unit) and the image plane. A frame member that holds alens may be designed to serve as the flare stop. Alternatively, aseparate flare stop member may be provided. The flare stop may be formedon a lens surface of any one of the lenses in the zoom lens by directprinting, coating or by sticking a sheet. The opening of the flare stopmay have various shapes such as circular, elliptical, rectangular, orpolygonal shape, or the opening shape may be defined by a curvespecified by a mathematical function. The flare stop may cut not onlydetrimental beams but also beams that may cause coma flare in theperipheral regions of the image.

Anti-reflection coating may be applied on each of the lenses to reduceghost and flare. Multi-coating is preferable, since effective ghost andflare reduction can be expected. Furthermore, infrared cut coating maybe applied on a lens surface or on a surface of a cover glass etc.

It is preferred that focusing be performed by moving the fourth lensunit (or the rear lens unit).

When focusing is performed by moving the fourth lens unit (or the rearlens unit), load on the motor will be small, since the fourth lens unit(or the rear lens unit) is light in weight. This also makes it easier tosuppress changes in the angle of field with focusing operation.

Furthermore, this is advantageous in making the lens frame compact,since the entire length of the zoom lens does not change with focusingand a drive motor can be provided inside the lens frame.

Although it is preferred that focusing be performed by the fourth lensunit (or the rear lens unit) as described above, focusing may beperformed by the first, second or third lens unit. Alternatively,focusing may be performed by moving a plurality of lens units.Alternatively, focusing may be performed by moving the zoom lens as awhole. Alternatively, focusing may be performed by shifting a part ofthe lenses forward or backward.

Shading in the peripheral regions of the image may be reduced byshifting microlenses corresponding to individual light receiving pixelson the light receiving surface of the image pickup element. For example,design of microlenses may be varied in accordance with the incidenceangle of rays at the corresponding image height. A decrease in thebrightness or light quantity in the peripheral regions of the image maybe compensated by image processing.

In the case where the zoom lens has a focusing function, the conditionalexpressions presented above should be interpreted as conditions in thestate in which the zoom lens is focused on the farthest object point.

It is more preferred that some of the above described features beadopted in combination.

It is preferred that the image pickup apparatus have an imagetransformation section that transforms, by image processing, anelectrical signal containing a distortion due to the zoom lens into animage signal in which the distortion is corrected. This is advantageousin further reducing the size and the number of lenses in the zoom lens,since distortion of the image formed by the zoom lens is allowed.

In the case where the zoom lens has a focusing function, the conditionalexpressions presented above should be interpreted as conditions in thestate in which the zoom lens is focused on the farthest object point.

It is more preferred that some of the above described features beadopted in combination.

It is more preferred that the limit values in the conditionalexpressions be changed as follows.

As to conditional expression (1), it is more preferred that the lowerlimit value be 0.05, more preferably 0.09. It is more preferred that theupper limit value be 0.13, more preferably 0.12.

As to conditional expression (2), it is more preferred that the lowerlimit value be 0.20, more preferably 0.25. It is more preferred that theupper limit value be 0.45, more preferably 0.38, and still morepreferably 0.35.

As to conditional expression (3), it is more preferred that the lowerlimit value be 0.02, more preferably 0.045. It is more preferred thatthe upper limit value be 0.075, more preferably 0.06.

As to conditional expression (4), it is more preferred that the lowerlimit value be 1.30, more preferably 1.40. It is more preferred that theupper limit value be 2.50, more preferably 2.00.

As to conditional expression (5), it is more preferred that the lowerlimit value be 0.09. It is more preferred that the upper limit value be0.20.

As to conditional expression (6), it is more preferred that the lowerlimit value be 15, more preferably 18. It is more preferred that theupper limit value be 40, more preferably 30.

As to conditional expression (7), it is more preferred that the lowerlimit value be 1.2, more preferably 1.5. It is more preferred that theupper limit value be 1.8, more preferably 1.7.

As to conditional expression (8), it is more preferred that the lowerlimit value be 7.0. It is more preferred that the upper limit value be14.0, more preferably 11.2.

As to conditional expression (9), it is more preferred that the lowerlimit value be 0.60, more preferably 0.70. It is more preferred that theupper limit value be 0.95, more preferably 0.85.

It is more preferred that limit values in the conditional expressions bechanged as follows.

As to conditional expression (10), it is more preferred that the lowerlimit value be 0.15, more preferably 0.18. It is more preferred that theupper limit value be 0.27, more preferably 0.26.

As to conditional expression (11), it is more preferred that the lowerlimit value be 0.10, more preferably 0.12. It is more preferred that theupper limit value be 0.17, more preferably 0.15.

As to conditional expression (12), it is more preferred that the lowerlimit value be 0.03, more preferably 0.04. It is more preferred that theupper limit value be 0.07, more preferably 0.06, and still morepreferably 0.055.

As to conditional expression (13), it is more preferred that the lowerlimit value be 0.30, more preferably 0.50. It is more preferred that theupper limit value be 0.73, more preferably 0.70.

As to conditional expression (14), it is more preferred that the lowerlimit value be 0.09. It is more preferred that the upper limit value be0.20.

As to conditional expression (15), it is more preferred that the lowerlimit value be 1.30, more preferably 1.40. It is more preferred that theupper limit value be 2.50, more preferably 2.00.

In the above-described modes of the invention, it is more preferred thatsome of the conditions be satisfied simultaneously. In the morepreferred numerical range limitations by each of the conditionalexpressions presented above, limitation by only the upper limit value orthe lower limit value may be applied. Furthermore, the various featuresdescribed above may be adopted in any possible combination.

It is preferred, from the viewpoint of aberration correction, that thetotal number of lenses included in the zoom lens be 9 or more.

It is preferred, in order to achieve reduction in the size and cost,that the total number of lenses included in the zoom lens be 11 or less,more preferably 10 or less.

In the above-described modes of the invention, it is more preferred thatsome of the conditions be satisfied simultaneously. In the morepreferred numerical range limitations by each of the conditionalexpressions presented above, limitation by only the upper limit value orthe lower limit value may be applied. Furthermore, the various featuresdescribed above may be adopted in any possible combination.

In the following, some exemplary zoom lenses as embodiments of thepresent invention will be described. These zoom lenses are small in sizewhile having a high zoom ratio and a wide angle of field to meet thedemands of users who wish to use a zoom lens in extended shootingsituations without losing portability thereof. The zoom lenses accordingto the embodiments are inexpensive zoom lenses that provide good imagequality and are suitable for use with an electronic image pickup elementsuch as a CCD or CMOS sensor.

In the following, embodiments of the zoom lens and the image pickupapparatus according to the present invention will be described in detailwith reference to the drawings. It should be understood, however, thatthe embodiments are not intended to limit the present invention.

In the zoom lenses according to the first to third embodiments, theeffective image pickup area has a rectangular shape constantly at allthe zoom positions. The values associated with conditional expressionspresented below for each embodiment are for the state in which the zoomlens is focused on an object point at infinity.

In the embodiments, focusing is performed by moving the fourth lensunit, and focusing operation from an object point at a long distance toan object point at a short distance is performed by moving the fourthlens unit toward the object side.

Plane parallel plates include a low pass filter having IR cut coatingapplied thereon and a CCD cover glass. In the case where there is onlyone plane parallel plate, it is a CCD cover glass. In the case where thenumber of pixels is large, moire is not conspicuous even without a lowpass filter. The IR cut coating may be applied on a lens surface or thesurface of the cover glass. A shutter is provided at a position near thestop (iris stop) indicated in the lens data, though the shutter is notindicated in the numerical data. Alternatively, the same member mayserve as both the shutter and the stop.

In the following, the first to third embodiments of the zoom lensaccording to the present invention will be described. FIGS. 1A to 1E, 2Ato 2E and 3A to 3E are cross sectional views showing the configurationsof the zoom lenses according to the first to third embodiments at thewide angle end, in a first intermediate focal length state, in a secondintermediate focal length state, in a third intermediate focal lengthstate and at the telephoto end in the state in which the zoom lens isfocused on an object point at infinity. In FIGS. 1A through 3E, thefirst lens unit is denoted by G1, the second lens unit is denoted by G2,the iris stop (or aperture stop) is denoted by S, the third lens unit isdenoted by G3, the fourth lens unit is denoted by G4, a plane parallelplate having wavelength range restriction coating applied thereon thatblocks or reduces infrared light to constitute the low pass filter asdescribed above is denoted by F, a plane parallel plate constituting acover glass for an electronic image pickup element is denoted by C, andthe image plane is denoted by I. The cover glass C may have multi-layercoating for wavelength range restriction applied on its surface. Thecover glass C may be designed to have a function of a low pass filter.

The front side lens unit includes the first lens unit G1 and the secondlens unit G2. The rear side lens unit includes the third lens unit G3and the fourth lens unit G4 (which corresponds to the rear lens unit).

All the numerical data presented below are for the state in which thezoom lens is focused on an object at infinity. In the numerical data,dimensions are in mm and angles are in degrees. The zoom data arepresented for the wide angle end (WE), the first intermediate focallength state (ST1), the second intermediate focal length state (ST2),the third intermediate focal length state (ST3) and telephoto end (TE).

As shown in FIGS. 1A to 1E, the zoom lens according to the firstembodiment has a first lens unit G1 having a positive refracting power,a second lens unit G2 having a negative refracting power, an iris stopS, a third lens unit G3 having a positive refracting power and a fourthlens unit G4 having a positive refracting power, which are arranged inthe mentioned order from the object side.

During zooming from the wide angle end to the telephoto end, the firstlens unit G1 moves only toward the object side, the second lens unit G2moves only toward the object side, the third lens unit G3 moves onlytoward the object side, and the fourth lens unit G4 moves along a locusthat is convex toward the image side.

The first lens unit G1 is composed, in order from the object side, of acemented lens composed of a negative meniscus lens having a convexsurface directed toward the object side and a positive meniscus lenshaving a convex surface directed toward the object side, and a positivemeniscus lens having a convex surface directed toward the object side.The second lens unit G2 is composed, in order from the object side, of anegative meniscus lens having a convex surface directed toward theobject side and a cemented lens composed of a biconvex positive lens anda biconcave negative lens. The third lens unit G3 is composed, in orderfrom the object side, of a biconvex positive lens and a cemented lenscomposed of a positive meniscus lens having a convex surface directedtoward the object side and a negative meniscus lens having a convexsurface directed toward the object side. The fourth lens unit G4 iscomposed of a biconvex positive lens.

Aspheric surfaces are used in the image side surface of the positivemeniscus lens having a convex surface directed toward the object sidelocated closest to the image side in the first lens unit G1, both theside surfaces of the negative meniscus lens having a convex surfacedirected toward the object side in the second lens unit G2, the imageside surface of the biconcave negative lens located on the image side inthe second lens unit G2, both the side surfaces of the biconvex positivelens in the third lens unit G3 and both the side surfaces of thebiconvex positive lens in the fourth lens unit G4, namely there areeight aspheric surfaces in total.

As shown in FIGS. 2A to 2E, the zoom lens according to the secondembodiment has a first lens unit G1 having a positive refracting power,a second lens unit G2 having a negative refracting power, an iris stopS, a third lens unit G3 having a positive refracting power and a fourthlens unit G4 having a positive refracting power, which are arranged inthe mentioned order from the object side.

During zooming from the wide angle end to the telephoto end, the firstlens unit G1 moves only toward the object side, the second lens unit G2moves only toward the object side, the third lens unit G3 moves onlytoward the object side, and the fourth lens unit G4 moves along a locusthat is convex toward the image side.

The first lens unit G1 is composed, in order from the object side, of acemented lens composed of a negative meniscus lens having a convexsurface directed toward the object side and a positive meniscus lenshaving a convex surface directed toward the object side, and a biconvexpositive lens. The second lens unit G2 is composed, in order from theobject side, of a negative meniscus lens having a convex surfacedirected toward the object side and a cemented lens composed of abiconvex positive lens and a biconcave negative lens. The third lensunit G3 is composed, in order from the object side, of a biconvexpositive lens and a cemented lens composed of a positive meniscus lenshaving a convex surface directed toward the object side and a negativemeniscus lens having a convex surface directed toward the object side.The fourth lens unit G4 is composed of a biconvex positive lens.

Aspheric surfaces are used in the image side surface of the positivemeniscus lens having a convex surface directed toward the object sidelocated on the image side in the first lens unit G1, both the sidesurfaces of the negative meniscus lens having a convex surface directedtoward the object side in the second lens unit G2, the image sidesurface of the biconcave negative lens in the second lens unit G2, boththe side surfaces of the biconvex positive lens in the third lens unitG3 and both the side surfaces of the biconvex positive lens in thefourth lens unit G4, namely there are eight aspheric surfaces in total.

As shown in FIGS. 3A to 3E, the zoom lens according to the thirdembodiment has a first lens unit G1 having a positive refracting power,a second lens unit G2 having a negative refracting power, an iris stopS, a third lens unit G3 having a positive refracting power and a fourthlens unit G4 having a positive refracting power, which are arranged inthe mentioned order from the object side.

During zooming from the wide angle end to the telephoto end, the firstlens unit G1 moves only toward the object side, the second lens unit G2moves along a locus that is convex toward the object side, the thirdlens unit G3 moves only toward the object side, and the fourth lens unitG4 moves first toward the image side and thereafter reverses itsmovement direction twice.

The first lens unit G1 is composed, in order from the object side, of anegative meniscus lens having a convex surface directed toward theobject side and a biconvex positive lens. The second lens unit G2 iscomposed, in order from the object side, of a negative meniscus lenshaving a convex surface directed toward the object side and a cementedlens composed of a positive meniscus lens having a convex surfacedirected toward the image side and a biconcave negative lens. The thirdlens unit G3 is composed, in order from the object side, of a biconvexpositive lens and a cemented lens composed of a positive meniscus lenshaving a convex surface directed toward the object side and a negativemeniscus lens having a convex surface directed toward the object side.The fourth lens unit G4 is composed of a biconvex positive lens.

Aspheric surfaces are used in the image side surface of the biconvexpositive lens in the first lens unit G1, both the side surfaces of thenegative meniscus lens having a convex surface directed toward theobject side in the second lens unit G2, the image side surface of thebiconcave negative lens in the second lens unit G2, both the sidesurfaces of the biconvex positive lens in third lens unit G3 and boththe side surfaces of the biconvex positive lens in the fourth lens unitG4, namely there are eight aspheric surfaces in total.

Numerical data of each embodiment described above is shown below. Apartfrom symbols described above, f denotes a focal length of the entirezoom lens system, F_(NO) denotes an F number, ω denotes a half imageangle, WE denotes a wide angle end, ST denotes an intermediate state, TEdenotes a telephoto end, each of r1, r2, . . . denotes radius ofcurvature of each lens surface, each of d1, d2, . . . denotes a distancebetween two lenses, each of nd1, nd2, . . . denotes a refractive indexof each lens for a d-line, and each of νd1, νd2, . . . denotes an Abbe'snumber for each lens.

The overall length of the lens system which will be described later is alength which is obtained by adding the back focus to a distance from thefirst lens surface up to the last lens surface. BF (back focus) is aunit which is expressed upon air conversion of a distance from the lastlens surface up to a paraxial image plane.

When x is let to be an optical axis with a direction of traveling oflight as a positive (direction), and y is let to be in a directionorthogonal to the optical axis, a shape of the aspheric surface isdescribed by the following expression.

x=(y ² /r)/[1+{1−(K+1)(y/r)²}^(1/2) ]+A ₄ y ⁴ +A ₆ y ⁶ +A ₈ y ⁸ +A ₁₀ y¹⁰ +A ₁₂ y ¹²

where, r denotes a paraxial radius of curvature, K denotes a conicalcoefficient, A4, A6, A8, A10, and A₁₂ denote aspherical surfacecoefficients of a fourth order, a sixth order, an eight order, a tenthorder, and a twelfth order respectively. Moreover, in the asphericalsurface coefficients, ‘e−n’ (where, n is an integral number) indicates‘10^(−n)’.

Example 1 Unit Mm

Surface data Surface No r d nd νd  1 71.917 0.80 1.92286 18.90  2 37.5294.00 1.61800 63.33  3 138.659 0.10  4 26.623 5.50 1.77250 49.60  5*192621.886 Variable  6* 47.954 0.80 1.83481 42.71  7* 6.341 2.50  8118.332 1.60 1.94595 17.98  9 −10.823 0.80 1.83481 42.71 10* 14.081Variable 11(S) ∞ 0.30 12* 5.438 2.96 1.49700 81.54 13* −20.223 0.10 146.668 1.50 1.77250 49.60 15 11.000 0.70 1.84666 23.78 16 3.966 Variable17* 31.214 1.90 1.74330 49.33 18* −9.711 Variable 19 ∞ 0.40 1.5477162.84 20 ∞ 0.50 21 ∞ 0.50 1.51633 64.14 22 ∞ 0.37 Image plane (Lightreceiving surface)

Aspherical Coefficients

1th surface

5th surfacek=0.654, A4=4.76005e−06, A6=−5.27460e−10,A8=−8.76612e−12, A10=1.17810e−146th surfacek=0.000, A4=−2.34324e−04, A6=−3.13708e−06,A8=2.43681e−07, A10=−2.99207e−097th surfacek=0.369, A4=1.12943e−04, A6=1.11887e−05,A8=−1.27867e−06, A10=4.22626e−0810th surfacek=0.000, A4=−9.38012e−04, A6=−1.97637e−05,A8=2.81611e−06, A10=−1.02669e−0712th surfacek=0.000, A4=−5.69400e−04, A6=1.16631e−05,A8=−1.28093e−06, A10=4.10229e−0713th surfacek=0.000, A4=8.40292e−04, A6=3.86549e−05,A8=−4.12799e−06, A10=8.33733e−0717th surfacek=0.772, A4=−2.37726e−04, A6=1.50005e−05,A8=−2.25542e−07, A10=6.46375e−1018th surfacek=0.000, A4=9.09060e−05, A6=1.72680e−05,A8=−2.44150e−07

Zoom data WE ST1 ST2 ST3 TE focal length 4.85 10.51 19.35 47.08 100.00Fno. 8.00 8.00 8.00 8.00 8.00 2ω (°) 80.53 37.80 21.98 9.85 4.60 BF 5.684.90 4.41 1.76 2.46 Total length 48.06 59.63 65.78 70.32 74.55 d5 0.2010.00 15.08 18.40 20.19 d10 12.73 10.77 9.35 7.23 1.00 d16 5.89 10.4113.38 19.37 27.34 d18 4.23 3.44 2.95 0.30 1.00

Example 2 Unit Mm

Surface data Surface No r d nd νd  1 62.968 0.80 1.92286 18.90  2 34.4963.50 1.61800 63.33  3 155.107 0.10  4 29.363 4.00 1.77250 49.60  5*−3588.280 Variable  6* 59.714 0.80 1.83481 42.71  7* 6.851 2.50  8190.030 1.60 1.94595 17.98  9 −10.823 0.80 1.83481 42.71 10* 14.854Variable 11(S) ∞ 0.30 12* 5.209 2.30 1.49700 81.54 13* −15.384 0.10 146.338 1.50 1.77250 49.60 15 8.181 0.70 1.90000 23.80 16 3.690 Variable17* 39.261 1.90 1.74330 49.33 18* −10.366 Variable 19 ∞ 0.50 1.5163364.14 20 ∞ 0.77 Image plane (Light receiving surface)

Aspherical Coefficients

5th surfacek=0.654, A4=2.62061e−06, A6=2.58535e−09,A8=−1.90250e−11, A10=3.54790e−146th surfacek=0.000, A4=−2.25942e−04, A6=−2.43816e−06,A8=2.41139e−07, A10=−3.04832e−097th surfacek=0.736, A4=8.01450e−05, A6=1.05151e−05,A8=−1.27831e−06, A10=4.22643e−0810th surfacek=0.000, A4=−8.43392e−04, A6=−2.02943e−05,A8=2.81754e−06, A10=−1.02665e−0712th surfacek=0.000, A4=−6.79145e−04, A6=1.27390e−05,A8=−1.28037e−06, A10=4.10229e−0713th surfacek=0.000, A4=8.06000e−04, A6=3.79038e−05,A8=−4.12831e−06, A10=8.33733e−0717th surfacek=0.206, A4=−1.30715e−04, A6=1.46119e−05,A8=−2.28610e−07, A10=6.27096e−1018th surfacek=0.000, A4=9.09060e−05, A6=1.76840e−05,A8=−2.41180e−07

Zoom data WE ST1 ST2 ST3 TE focal length 4.64 9.70 18.11 47.11 85.12Fno. 8.00 8.00 8.00 8.00 8.00 2ω (°) 85.45 40.52 22.84 9.50 5.15 BF 5.464.52 3.90 1.15 2.75 Total length 43.70 54.19 60.86 67.07 71.338 d5 0.208.87 14.21 18.10 20.67 d10 12.77 10.46 8.59 5.72 1.00 d16 4.37 9.4413.25 21.20 26.01 d18 4.36 3.42 2.80 0.05 1.65

Example 3 Unit Mm

Surface data Surface No r d nd νd  1 47.985 0.80 1.92286 18.90  2 28.2430.05  3 21.248 4.08 1.77250 49.60  4* −116.425 Variable  5* 165.072 0.801.83481 42.71  6* 7.304 2.03  7 −91.810 1.91 1.94595 17.98  8 −10.8230.80 1.83481 42.71  9* 17.995 Variable 10(S) ∞ 0.30 11* 5.280 2.461.49700 81.54 12* −14.512 0.28 13 6.294 1.52 1.77250 49.60 14 8.181 0.802.00068 25.47 15 3.729 Variable 16* 473.250 3.74 1.74330 49.33 17*−11.842 Variable 18 ∞ 0.40 1.54771 62.84 19 ∞ 0.50 20 ∞ 0.50 1.5163364.14 21 ∞ 0.37 Image plane (Light receiving surface)

Aspherical Coefficients

4th surfacek=0.654, A4=1.17011e−05, A6=−1.96762e−09,A8=−8.17022e−12, A10=5.12387e−155th surfacek=0.000, A4=−8.24711e−05, A6=−2.76503e−06,A8=2.27122e−07, A10=−3.15083e−096th surfacek=0.787, A4=6.34033e−05, A6=1.04135e−05,A8=−1.27967e−06, A10=4.22656e−089th surfacek=0.000, A4=−5.37026e−04, A6=−1.90025e−05,A8=2.82021e−06, A10=−1.02639e−0711th surfacek=0.000, A4=−4.84682e−04, A6=1.18845e−05,A8=−1.28144e−06, A10=4.10225e−0712th surfacek=0.000, A4=8.84774e−04, A6=3.84287e−05,A8=−4.12823e−06, A10=8.33734e−0716th surfacek=0.000, A4=9.93673e−21, A6=1.56889e−05,A8=−2.27236e−07, A10=6.31860e−1017th surfacek=0.000, A4=9.09060e−05, A6=1.67850e−05,A8=−2.42650e−07

Zoom data WE ST1 ST2 ST3 TE focal length 4.84 9.46 19.43 38.31 100.77Fno. 3.60 4.82 5.12 5.60 8.16 2ω (°) 84.84 44.20 22.40 11.52 4.50 BF5.74 5.42 4.57 6.55 2.64 Total length 43.66 50.40 59.78 67.75 69.21 d40.06 5.04 11.53 16.83 21.12 d9 14.64 10.80 8.50 5.21 1.27 d15 3.65 9.5715.61 19.60 24.62 d17 4.28 3.96 3.11 5.09 1.18

In fourth to sixth embodiments, the zoom lenses according to the firstto third embodiments are respectively used in an image pickup apparatusthat has a function of correcting distortion electrically, wherein theshape of the effective image pickup area is changed upon zooming.Therefore, in the thirteenth to fourth to sixth embodiments, the imageheight and the angle of field at a zoom position are different fromthose in the respective corresponding embodiments.

In the fourth to sixth embodiments, barrel distortion (hat shaped) thatoccurs at wide angle positions is corrected electrically, and athus-corrected image is recorded or displayed.

In the zoom lenses according to the embodiments, barrel occurs on therectangular photoelectric conversion surface, at the wide angle end.Whereas, at the telephoto end and near the intermediate focal lengthstate, distortion is suppressed.

To correct distortion electrically, the effective image pickup area isdesigned to have a barrel shape at the wide angle end and a rectangularshape near the intermediate focal length position and at the telephotoend. In addition, the effective image pickup area, which has been set inadvance, is transformed into rectangular image information with reduceddistortion by image transformation using image processing.

The maximum image height IH_(w) at the wide angle end is designed to besmaller than the maximum image height IH_(s) at the intermediate focallength state and the maximum image height IH_(t) at the telephoto end.

In the fourth to sixth embodiments, the effective image pickup area isdesigned in such a way that the effective image pickup area at the wideangle end has a dimension in the shorter side direction equal to thedimension in the shorter side direction of the photoelectric conversionsurface, and a distortion of approximately −3% remains after imageprocessing.

At an angle of field of 28 mm in 135 mm film size, it is preferable thata distortion of approximately −3% remains in consideration of balancebetween an influence of perspective at a time of image taking forthree-dimensional object and an influence of barrel shape distortion.

As a matter of course, a barrel shaped area smaller than that describedabove may be set as the effective image pickup area, and image resultingfrom transformation of this area into a rectangular area may berecorded/reproduced.

The zoom lens used in the fourth embodiment is the same as the zoom lensaccording to the first embodiment.

The zoom lens used in the fifth embodiment is the same as the zoom lensaccording to the second embodiment.

The zoom lens used in the sixth embodiment is the same as the zoom lensaccording to the third embodiment.

Data of image height and total image angle in example 4 are as shownbelow.

Zoom data WE ST1 ST2 ST3 TE focal length 4.85 19.35 100.00 10.51 47.08Fno. 8.00 8.00 8.00 8.00 8.00 2ω (°) 80.12 37.80 21.98 9.85 4.60 Totallength 3.847 3.88 3.88 3.88 3.88

Data of image height and total image angle in example 5 are as shownbelow.

Zoom data WE ST1 ST2 ST3 TE focal length 4.64 18.11 85.12 9.70 47.11Fno. 8.00 8.00 8.00 8.00 8.00 2ω (°) 82.846 40.518 22.838 9.502 5.146Total length 3.60 3.80 3.80 3.80 3.80

Data of image height and total image angle in example 6 are as shownbelow.

Zoom data WE ST1 ST2 ST3 TE focal length 4.84 9.46 19.43 38.31 100.77Fno. 3.60 4.82 5.12 5.60 8.16 2ω (°) 82.078 44.200 22.400 11.518 4.498Total length 3.69 3.88 3.88 3.88 3.88

Aberration diagrams of the zoom lenses according to the first to thirdembodiments in the state in which the zoom lenses are focused on anobject point at infinity are shown in FIGS. 4A to 9E.

FIGS. 4A, 6A, and 8A show spherical aberration (SA), astigmatism (AS),distortion (DT) and chromatic aberration of magnification (CC) at thewide angle end.

FIGS. 4B, 6B, and 8B show spherical aberration (SA), astigmatism (AS),distortion (DT) and chromatic aberration of magnification (CC) in afirst intermediate state.

FIGS. 4C, 6C, and 8C show spherical aberration (SA), astigmatism (AS),distortion (DT) and chromatic aberration of magnification (CC) at asecond intermediate end.

FIGS. 5D, 7D, and 9D show spherical aberration (SA), astigmatism (AS),distortion (DT) and chromatic aberration of magnification (CC) at athird intermediate end.

FIGS. 5E, 7E, and 9E show spherical aberration (SA), astigmatism (AS),distortion (DT) and chromatic aberration of magnification (CC) at thetelephoto end.

In the aberrations diagrams, the sign “co” represents half the angle offield.

As shown in FIGS. 10A, 10B and 10C, when the zoom lenses of the first,second and third embodiments are collapsed, the first lens unit G1, thesecond lens unit G2 and the fourth lens unit G4 move along the opticalaxis of the lens units toward the image side, and the third lens unitmoves away from the optical axis to a position behind the first lensunit G1 to stay side by side with the second lens unit G2 and the fourthlens unit G4.

Expressional condition values are shown as below:

Example 1 Example 2 Example 3 (1) f₃/f_(t) 0.110 0.120 0.100 (2)f₁/f_(t) 0.346 0.420 0.340 (3) |f₂|/f_(t) 0.058 0.071 0.058 (4) ΣD1/ΣD21.825 1.474 0.891 (5) f₄/f_(t) 0.102 0.132 0.155 (6) Zooming ratiof_(t)/f_(w) 20.637 18.362 20.832 (7) L_(t)/L_(w) 1.548 1.630 1.581 (8)F_(t) 8.000 8.000 8.158 (9) IH_(w)/f_(w) 0.801 0.820 0.802 total numberof lenses 10 10 9 IH 3.88 3.8 3.88 f1 34.617 35.753 34.313 f2 −5.813−6.070 −5.812 f3 10.999 10.184 10.096 f4 (fRL) 10.166 11.216 15.595Example 4 Example 5 Example 6 (1) f₃/f_(t) 0.110 0.120 0.100 (2)f₁/f_(t) 0.346 0.420 0.340 (3) |f₂|/f_(t) 0.058 0.071 0.058 (4) ΣD1/ΣD21.825 1.474 0.891 (5) f₄/f_(t) 0.102 0.132 0.155 (6) Zooming ratiof_(t)/f_(w) 20.637 18.362 20.832 (7) L_(t)/L_(w) 1.548 1.630 1.581 (8)F_(t) 8.000 8.000 8.158 (9) IH_(w)/f_(w) 0.794 0.777 0.763 Total numberof lenses 10 10 9 f1 34.617 35.753 34.313 f2 −5.813 −6.070 −5.812 f310.999 10.184 10.096 f4 (fRL) 10.166 11.216 15.595

Further, conditional expression values are shown as below:

Example 1 Example 2 Example 3 (10) (ΣD_(1-R))/f_(t) 0.233 0.242 0.191(11) (ΣD_(12R))/f_(t) 0.180 0.188 0.141 (12) Σd₃/f_(t) 0.053 0.054 0.050(13) Δ_(1G2G3G)/(f_(t) − f_(w)) 0.538 0.668 0.499 (14) f_(RL)/f_(t)0.102 0.132 0.155 (15) ΣD1/ΣD2 1.825 1.474 0.891 Example 4 Example 5Example 6 (10) (ΣD_(1-R))/f_(t) 0.233 0.242 0.191 (11) (ΣD_(12R))/f_(t)0.180 0.188 0.141 (12) Σd₃/f_(t) 0.053 0.054 0.050 (13)Δ_(1G2G3G)/(f_(t) − f_(w)) 0.538 0.668 0.499 (14) f_(RL)/f_(t) 0.1020.132 0.155 (15) ΣD1/ΣD2 1.825 1.474 0.891

Incidentally, for preventing the occurrence of the ghost and the flare,generally, the antireflection coating is applied to a surface of a lensin contact with air.

On the other hand, at a cemented surface of a cemented lens, arefractive index of an adhesive is sufficiently higher than a refractiveindex of air. Therefore, in many cases, a reflectance is originally ofthe level of a single-layer coating, or lower, and the coating isapplied in few cases. However, when the antireflection coating isapplied positively even to the cemented surface, it is possible toreduce further the ghost and the flare, and to achieve a more favorableimage.

Particularly, recently, a glass material having a high refractive indexhas been widely used in an optical system of cameras, for having a higheffect on the aberration correction. However, when the glass materialhaving a high refractive index is used as a cemented lens, a reflectionat the cemented surface becomes unignorable. In such a case, applyingthe antireflection coating on the cemented surface is particularlyeffective.

An effective usage of the cemented surface coating has been disclosed inJapanese Patent Application Laid-open Publication No. Hei 2-27301, No.2001-324676, No. 2005-92115, and U.S. Pat. No. 7,116,482. In thesepatent literatures, a cemented lens surface coating in a first lens unitof a positive preceding zoom lens system has been described, and thesame as disclosed in these patent literatures may be implemented for thecemented lens surface in the first lens unit having a positive power, ofthe present invention.

As a coating material to be used, according to a refractive index of theadhesive material and a refractive index of the lens which is a base,coating materials such as Ta₂O₅, TiO₂, Nb₂O₅, ZrO₂, HfO₂, CeO₂, SnO₂,In₂O₃, ZnO, and Y₂O₃ having a comparatively higher refractive index, andcoating materials such as MgF₂, SiO₂, and Al₂O₃ having a comparativelylower refractive index may be selected appropriately, and set to a filmthickness which satisfies phase conditions.

Naturally, similar to the coating on the surface of the lens in contactwith air, the coating on the cemented surface may also be let to be amulti layer coating. By combining appropriately a film thickness and acoating material of number of films not less than in two layers, it ispossible to reduce further the reflectance, and to control spectralcharacteristics and angular characteristics.

Moreover, it is needless to mention that for the cemented surface oflenses other than the lenses in the first lens unit, it is effective toapply the coating on the cemented surface based on a similar idea.

(Correction of Distortion)

Incidentally, when the zoom lens system of the present invention isused, a digital correction of distortion of an image is carried outelectrically. A basic concept for the digital correction of thedistortion of an image will be described below.

For example, as shown in FIG. 11, with a point of intersection of anoptical axis and an image pickup plane to be a center, a magnificationon a circumference (image height) of a circle of radius R making acontact internally with a longer side of an effective image pickup planeis fixed, and this circumference is let to be a base reference for thecorrection. Next, each point on a circumference (image height) of anarbitrary radius r(ω) other than the radius R is moved in a substantialdirection of radiation, and the correction is carried out by moving on aconcentric circle such that the radius becomes r′(ω).

For example, in FIG. 11, a point P₁ on a circumference of an arbitraryradius r₁(ω) positioned at an inner side of a circle of radius R ismoved to a point P₂ on a circumference of a radius r₁′(ω) which is to becorrected, directed toward a center of the circle. Moreover, a point Q₁on a circumference of an arbitrary radius r₂(ω) positioned at an outerside of the circle of radius R is moved to a point Q₂ on a circumferenceof a radius r₂′(ω) which is to be corrected, directed toward a directionaway from the center of the circle.

Here, r′(ω) can be expressed as follows.

r′(ω)=α·f·tan ω(0≦α≦1)

where, ω is a half image angle of an object and f is a focal length ofan imaging optical system (the zoom lens system in the presentinvention).

Here, when an ideal image height corresponding to a circle (imageheight) of radius R is let to be Y, then

α=R/Y═R/(f·tan ω).

The optical system, ideally, is rotationally symmetric with respect toan optical axis. In other words, the distortion also occurs in arotationally symmetric manner with respect to the optical axis.Consequently, as it has been described above, in a case of correctingelectrically the optical distortion, when it is possible to carry outcorrection by fixing a magnification on a circumference (image height)of the circle of radius R making a contact internally with a longer sideof the effective image pickup plane, with a point of intersection of anoptical axis on a reproduced image, and an image pickup plane to be acenter, and moving each point on the circumference (image height) ofradius r(ω) other than the radius R in a substantial direction ofradiation, and moving on a concentric circle such that the radiusbecomes r′(ω), it can be considered to be advantageous from a point ofamount of data and amount of calculation.

Incidentally, an optical image ceases to be a continuous amount at apoint of time when an image is picked up by an electronic image pickupelement (due to sampling). Consequently, the circle of radius R which isdrawn exactly on the optical image ceases to be an accurate circle aslong as pixels on the electronic image pickup element are not arrangedradially.

In other words, regarding a shape correction of image data expressed foreach discrete coordinate point, a circle which can fix the magnificationdoes not exist. Therefore, for each pixel (Xi, Yj), a method ofdetermining coordinates of a destination of movement (Xi′, Yj′) may beused. When two or more points (Xi, Yj) have moved to the coordinates(Xi′, Yj′), an average of values of each pixel is taken. Moreover, whenthere is no point which has moved, interpolation may be performed byusing a value of coordinate (Xi′, Yj′) of some of the surroundingpixels.

Such method is effective for correction when the distortion with respectto the optical axis is remarkable due to a manufacturing error etc. ofthe optical system or the electronic image pickup element, in theelectronic image pickup apparatus having the zoom lens system inparticular, and when the circle of the radius R drawn on the opticalimage is asymmetric. Moreover, it is effective for correction when thereoccurs to be a geometric distortion at the time of reproducing a signalto an image in an image pickup element or various output devices.

In the electronic image pickup apparatus of the present invention, forcalculating a correction amount r′(ω)−r(ω), an arrangement may be madesuch that a relationship between r(ω), in other words, half image angleand the image height, or a relationship between a real image height rand an ideal image height r′/α is recorded in a recording medium whichis built-in in the electronic image pickup apparatus.

For an image after the distortion correction, not to have an extremeshortage of an amount of light at both ends in a direction of shortside, the radius R may satisfy the following conditional expression.

0≦R≦0.6Ls

where, Ls is a length of a short side of the effective image pickupsurface.

It is preferable that the radius R satisfies the following conditionalexpression.

0.3Ls≦R≦0.6Ls

Furthermore, it is most advantageous to match the radius R with a radiusof a circle making an internal contact in a short side direction of asubstantially effective image pickup plane. In a case of correction inwhich, the magnification is fixed near the radius R=0, in other words,near on the axis, it is somewhat disadvantageous from an aspect ofsubstantial number of images, but it is possible to secure an effect formaking the size small even when the angle is widened.

A focal length interval which requires a correction is divided into anumber of focal point zones. Moreover, the correction may be carried outwith the amount of correction as in a case in which, a correction resultwhich satisfies substantially the following relationship

r′(ω)=α·f·tan ω

near a telephoto end in the focal point zones which are divided.

However, in this case, at a wide angle end in the focal point zoneswhich are divided, a barrel-shape distortion at the wide angle end ofthe focal point zones which are divided is remained to some extent.Moreover, when the number of divided zones is increased, there arises aneed to hold specific data necessary for correction, additionally in arecording medium. Therefore it is not preferable to increase the numberof divided zones. Therefore, one or a plurality of coefficientsassociated with each focal length in the focal point zones which aredivided, are calculated in advance. The coefficients may be determinedbased on a measurement by simulation or by actual equipment.

An amount of correction in a case in which, the correction result whichsatisfies substantially the following relationship

r′(ω)=αf·tan ω

near the telephoto end in the focal point zones which are divided may becalculated, and may let to be a final amount of correction bymultiplying uniformly the coefficient for each focal length with respectto this amount of correction.

Incidentally, when there is no distortion in an image achieved byimaging (forming an image) of an infinite object, the followingrelationship

f=y/tan ω

holds.

Here, y denotes a height (image height) of an image point from theoptical axis, f denotes a focal length of an imaging system (zoom lenssystem in the present invention), and ω denotes an angle (object halfimage angle) with respect to the optical axis in an object pointdirection corresponding to image points connecting from a center on animage pickup plane up to a position of y.

When there is a barrel-shape distortion in the imaging system, therelationship becomes

f>y/tan ω.

In other words, when the focal length f of the imaging system, and theimage height y are let to be fixed, a value of ω becomes large.

(Digital Camera)

FIG. 12 to FIG. 14 are conceptual diagrams of a structure of a digitalcamera according to the present invention in which a zoom lens systemdescribed above is incorporated in a taking optical system 141. FIG. 12is a front perspective view showing an appearance of a digital camera140, FIG. 13 is a rear perspective view of the same, and FIG. 14 is aschematic cross-sectional view showing a structure of the digital camera140. In FIG. 12 and FIG. 14, show an uncollapsed state of the takingoptical system 141. The digital camera 140, in a case of this example,includes the taking optical system 141 having a taking optical path 142,a finder optical system 143 having a finder optical path 144, a shutterbutton 145, a flash 146, a liquid-crystal display monitor 147, afocal-length changing button 161, and a setting changing switch 162 etc.and in the uncollapsed state of the taking optical system 141, bysliding a cover 160, the taking optical system 141, the finder opticalsystem 143, and the flash 146 are covered by the cover 160. Further,when the cover 160 is opened and the digital camera is set in a phototaking state, the taking optical system 141 assumes the uncollapsedstate as shown in FIG. 12, when the shutter button 145 disposed on anupper portion of the digital camera 140 is pressed, in synchronizationwith the pressing of the shutter button 145, a photograph is taken bythe taking optical system 141 such as the zoom lens system in the firstembodiment. An object image formed by the taking optical system 141 isformed on an image pickup surface of a CCD 149 via a cover glass C and alow pass filter on which a wavelength region restricting coating isapplied. An object image which is received as light by the CCD 149 isdisplayed on the liquid-crystal display monitor 147 which is provided ona rear surface of the digital camera 140 as an electronic image, via aprocessing means 151. Moreover, a recording means 152 is connected tothe processing means 151, and it is also possible to record theelectronic image which is taken. The recording means 152 may be providedseparately from the processing means 151, or may be formed by recordingby writing electronically in a flexible disc, a memory card, or an MOetc. Moreover, the camera may be formed as a silver-salt camera in whicha silver-salt film is disposed instead of the CCD 149.

Furthermore, a finder objective optical system 153 is disposed on thefinder optical path 144. The finder objective optical system 153consists of a plurality of lens units (three units in the diagram), andtwo prisms, and is made of a zoom optical system in which a focal lengthchanges in synchronization with a zoom lens system of the taking opticalsystem 141. An object image formed by the finder objective opticalsystem 153 is formed on a field frame 157 of an erecting prism 155 whichis an image erecting member. On a rear side of the erecting prism 155,an eyepiece optical system 159 which guides an erected image to aviewer's eyeball, is disposed. A cover member 150 is disposed on anemergence side of the eyepiece optical system 159.

Since the digital camera 140 structured in such manner has the takingoptical system 141 according to the present invention, has an extremelysmall thickness in collapsed state, and an extremely stable imagingperformance in the entire zooming region at high magnification, it ispossible to realize a high-performance, a small size, and a widening ofangle.

As per the above the zoom lens according to the present invention can beadvantageously applied to a zoom lens that can achieve both compactnessand high zoom ratio.

According to the invention, there can be provided a zoom lens that caneasily achieve compactness and high zoom ratio. There can also beprovided an image pickup apparatus equipped with such a zoom lens.

1. A zoom lens consisting, in order from an object side thereof, of: afront side lens unit having a negative refracting power at a wide angleend; and a rear side lens unit having a positive refracting power at thewide angle end, wherein the front side lens unit consists of a firstlens unit having a positive refracting power and a second lens unithaving a negative refracting power, the rear side lens unit consists ofa third lens unit having a positive refracting power and a rear lensunit on an image side of the third lens unit, during zooming from thewide angle end to the telephoto end, distances between the first lensunit, the second lens unit, the third lens unit and the rear lens unitrespectively change, the distance between the first lens unit and thesecond lens unit is larger at the telephoto end than at the wide angleend, the distance between the second lens unit and the third lens unitis smaller at the telephoto end than at the wide angle end, the distancebetween the third lens unit and the rear lens unit at the telephoto endis different from that at the wide angle end, the first lens unit islocated closer to the object side at the telephoto end than at the wideangle end, the third lens unit is located closer to the object side atthe telephoto end than at the wide angle end, and the zoom lenssatisfies the following condition:0.10<(ΣD _(1-R))/f _(t)<0.28  (10) where ΣD_(1-R) is the sum ofthicknesses, on the optical axis, of the first lens unit, the secondlens unit, the third lens unit and the rear lens unit, the thickness ofeach lens unit referring to an actual distance from an object sidesurface of the lens located closest to the object side in that lens unitto an image side surface of the lens located closest to the image sidein that lens unit, and f_(t) is a focal length of the entire zoom lenssystem at the telephoto end.
 2. The zoom lens according to claim 1,wherein the rear lens unit has a positive refracting power.
 3. The zoomlens according to claim 1, wherein a focal length of the rear lens unitis constant during zooming from the wide angle end to the telephoto end.4. The zoom lens according to claim 1, which satisfies the followingcondition:0.03<Σd ₃ /F _(t)<0.075  (12) where Σd₃ is a thickness of the third lensunit on the optical axis.
 5. The zoom lens according to claim 1, whereinthe first lens unit, the second lens unit and the third lens unitsatisfy the following condition:0.10<Δ_(1G2G3G)/(f _(t) −f _(w))<0.77  (13) where Δ_(1G2G3G) is the sumof the absolute values of displacements of the positions of the firstlens unit, the second lens unit and the third lens unit at the wideangle end from the positions of the respective lens units at the wideangle end, f_(w) is the focal length of the entire zoom lens system atthe wide angle end, and f_(t) is the focal length of the entire zoomlens system at the telephoto end.
 6. The zoom lens according to claim 1,wherein the second lens unit consists of two negative lenses and onepositive lens.
 7. The zoom lens according to claim 6, wherein the secondlens unit consists, in order from the object side, of a first negativelens, a positive lens and a second negative lens.
 8. The zoom lensaccording to claim 7, wherein an image side surface of the firstnegative lens in the second lens unit is a concave surface, an objectside surface of the second negative lens in the second lens unit is aconcave surface and an image side surface of the positive lens in thesecond lens unit is a convex surface.
 9. The zoom lens according toclaim 1 wherein the zoom lens is a four-unit zoom lens.
 10. The zoomlens according to claim 9, further comprising an iris stop disposedbetween an image side surface of the second lens unit and an image sidesurface of the third lens unit, wherein during zooming from the wideangle end to the telephoto end, the first lens unit moves in such a wayas to be located closer to the object side at the telephoto end than atthe wide angle end, the second lens unit moves, the third lens unitmoves in such a way as to be located closer to the object side at thetelephoto end than at the wide angle end, the rear lens unit moves, andthe iris stop moves.
 11. The zoom lens according to claim 1, wherein thefirst lens unit consists of a negative lens and a positive lens.
 12. Thezoom lens according to claim 1, which satisfies the following condition:9<f _(t) /f _(w)<50  (6) where f_(w) is the focal length of the entirezoom lens system at the wide angle end, and f_(t) is the focal length ofthe entire zoom lens system at the telephoto end.
 13. The zoom lensaccording to claim 1, which satisfies the following condition:1.1<L _(t) /L _(w)<2.0  (7) where L_(t) is an actual distance, on theoptical axis, from the lens surface closest to the object side in thefirst lens unit to an image plane at the telephoto end, and L_(w) is anactual distance, on the optical axis, from the lens surface closest tothe object side in the first lens unit to an image plane at the wideangle end.
 14. The zoom lens according to claim 1, wherein the firstlens unit satisfies the following condition:0.15<f ₁ /f _(t)<0.50  (2) where f₁ is a focal length of the first lensunit, and f_(t) is the focal length of the entire zoom lens system atthe telephoto end.
 15. The zoom lens according to claim 1, wherein thesecond lens unit satisfies the following condition:0.01<|f ₂ |/f _(t)<0.10  (3) where f₂ is a focal length of the secondlens unit, and f_(t) is the focal length of the entire zoom lens systemat the telephoto end.
 16. The zoom lens according to claim 1, whereinthe third lens unit satisfies the following condition:0.01<f ₃ /f _(t)<0.16  (1) where f₃ is a focal length of the third lensunit, and f_(t) is the focal length of the entire zoom lens system atthe telephoto end.
 17. The zoom lens according to claim 1, wherein therear lens unit satisfies the following condition:0.05<f _(RL) /f _(t)<0.30  (14) where f_(RL), is a focal length of therear lens unit at the wide angle end, and f_(t) is the focal length ofthe entire zoom lens system at the telephoto end.
 18. The zoom lensaccording to claim 17, wherein the rear lens unit moves in such a way asto be located closer to the image side at the telephoto end than at thewide angle end.
 19. The zoom lens according to claim 1, which satisfiesthe following condition at the telephoto end:5.6<F_(t)<16.0  (8) where F_(t) is an F-number of the zoom lens at thetelephoto end or the minimum F-number in the case where the F-number isvariable.
 20. An image pickup apparatus comprising a zoom lens accordingto claim 1, which satisfies the following condition:1.26<ΣD1/ΣD2<3.00  (15) where ΣD1 is a thickness of the first lens uniton the optical axis, and ΣD2 is a thickness of the second lens unit onthe optical axis, the thickness of each lens unit on the optical axisreferring to an actual distance from an object side surface of the lenslocated closest to the object side in the lens unit to an image sidesurface of the lens located closest to the image side in the lens unit.21. The zoom lens according to claim 1, wherein every lens unit includedin the zoom lens has an aspheric lens surface.
 22. The zoom lensaccording to claim 1, wherein the total number of lenses included in thezoom lens is 10, 11, 12, 13 or 14, each of the first and third lensunits comprises a negative lens and a positive lens, and the second lensunit comprises a positive lens and a plurality of negative lenses. 23.An image pickup apparatus comprising: a zoom lens according to claim 1;an image pickup element disposed on the image side of the zoom lens thatconverts an image formed by the zoom lens into an electrical signal. 24.The image pickup apparatus according to claim 23, further comprising animage processing section that performs signal processing that correctsaberration contained in the electrical signal.
 25. The image pickupapparatus according to claim 23, which satisfies the followingcondition:0.05<IH _(w) /f _(w)<1.00  (9) where IH_(w) is the maximum image heightat the wide angle end, and f_(w) is the focal length of the entire zoomlens system at the wide angle end.
 26. The image pickup apparatusaccording to claim 23, wherein the optical axis of the third lens unitis retracted to a position away from the optical axis of the first,second and rear lens units.